As consumer demand for wireless communications devices with extended battery life continues to increase, power dissipation has become a major concern for device manufacturers. A major contributor to the power dissipation of many wireless communications devices is the power amplifier. Because the efficiency of a power amplifier is inversely proportional to the power dissipation thereof, increasing the efficiency of a wireless communications device results in a decrease in the power dissipation of the device.
Current wireless communications standards require a wireless communications device to operate at a number of pre-defined output power levels. These power levels are selected by the wireless communications device depending on the propagation characteristics of the RF channel in use. For example, the 2G Enhanced Data Rates for GSM Evolution (EDGE) standard requires output power levels ranging from 5 dBm to 27 dBm in 2 dB steps for signals transmitted in the GSM-850 or GSM-900 frequency bands by a Class E2 mobile station and requires output power levels ranging from 0 dBm to 26 dBm in 2 dB steps in the Digital Communications Standard (DCS) and Personal Communications Standard (PCS) frequency bands by a Class E2 mobile station. In another example the 3G Wideband Code Division Multiple Access (WCDMA) standard requires output power levels ranging from less than −50 dBm to +27 dBm in steps as small as 1 dB for Class 2 User Equipment (UE).
Most modern cellular systems such as EDGE and WCDMA further require that the radio transmission system be reasonably linear. Thus the ratio of output power to input power, or gain, should be reasonably constant over the output power range traversed by the amplitude of the modulation signal.
One method of improving the efficiency of a power amplifier at low power levels is to reduce the quiescent current delivered to the power amplifier. Accordingly, many modern power amplifiers are designed to operate with several selectable DC bias levels that reduce the quiescent current as the output power level is reduced. However, as the quiescent current is reduced, the linearity of a power amplifier is degraded at higher power levels. One mechanism that degrades the linearity of the power amplifier is rectification of an RF input signal delivered to the power amplifier. Rectification of the RF input signal contributes to the DC bias level and thus increases the gain as the RF input signal level increases. This effect is typically referred to as “gain peaking,” and is a limiting factor in the efficiency achievable by conventional power amplifiers.
Accordingly, there is a present need for an RF power amplifier architecture that allows operation at low quiescent current levels while minimizing variations in gain.